1. Field of the Invention
The present invention relates to a printing apparatus wherein a ribbon holder for holding a ribbon or ribbons is initially placed in a predetermined initial position. More particularly, this invention relates to improvements relating to initialization of such printing apparatus wherein a print head is moved along a line of printing to a predetermined zero position while the ribbon holder is brought to a predetermined initial position.
2. Discussion of the Related Art
Usually, a ribbon holder used in a printer is adapted to support a plurality of ribbons including at least one printing ribbon for printing on a recording medium. The ribbons may include a correction ribbon for erasing printed characters. The ribbon holder has two more more operating positions which corresponds to the respective ribbons and one of which is selected as a predetermined initial position in which the ribbon holder is placed upon initialization of the printer. The operating positions are spaced from each other in a direction perpendicular to the line of printing. In each operating position of the ribbon holder, the active portion or length of the corresponding ribbon is aligned with a line of printing for printing or erasure.
In some printers, the ribbon holder has a plurality of operating positions which include at least one position corresponding to at least one ribbon, and at least one dummy position which does not correspond to any ribbon. A dummy position may be used as the predetermined initial position, so that the ribbon holder is initially placed in the dummy position in which none of the ribbon or ribbons is aligned with the line of printing. With the ribbon holder placed in the dummy position, a portion of the recording medium along the line of printing is visible.
In such printer, the print head is also initialized, namely, moved along the line of printing to a predetermined zero position, which is generally one of opposite extreme ends of a movement range of the print head over which the print head is reciprocable by a suitable feeding device.
An example of such printer in which the print head is initially returned to the zero position while the ribbon holder is brought to the predetermined initial position is disclosed in JP-A-63-165177. In this printer, the print head is reciprocable along a line of printing parallel to a platen, between left and right ends of a predetermined movement range, by a head feeding motor, so that printing is effected by the print head via a currently selected printing ribbon on a recording medium, over a predetermined printing range within the movement range. A desired one of two or more ribbons supported by the ribbon holder is selected by activating a motor of a ribbon selecting device to move the ribbon holder to a position corresponding to the desired ribbon, so that the active portion of the desired ribbon is aligned with the line of printing and located between the print head and the platen.
The printer disclosed in the above-identified publication includes a sensor which generates first and second signals when the printer is placed in respective first and second states. The first state is a state in which the print head is not located at the zero position (left or right end of the movement range), while the ribbon holder is placed in the predetermined initial position. The second state is a state in which the print head is located at the zero position, or a state in which the print head is not located at the zero position while the ribbon holder is not placed in the initial position.
The printer further includes initializing means for moving the print head to the zero position and bringing the ribbon holder to the predetermined initial position. If the first signal is initially generated by the sensor, the initializing means determines that the print head is not located at the zero position while the ribbon holder is placed in the initial position, and therefore commands the head feeding motor to return the print head to the zero position. When the second signal is generated by the sensor, the initializing means determines that the print head has been returned to the zero position with the ribbon holder held in the initial position, and terminates an initializing operation.
If the second signal is initially generated by the sensor, the initializing means determines that the print head is located at the zero position (pending the currently selected position of the ribbon holder), or that the print head is not located at the zero position while the ribbon holder is not placed in the initial position. In this case, the initializing means commands the head feeding motor to move the print head from either the zero position or a position other than the zero position, in a direction away from the zero position. If the second signal remains even after the head feeding motor has been commanded to move the print head in the direction away from the zero position, the initializing means determines that the ribbon holder is not placed in the initial position pending whether the print head is located at the zero position or not, and then commands the ribbon selecting motor to move the ribbon holder from the currently selected position to another. When the second signal is changed to the first signal, the initializing means determines that the ribbon holder is brought to the initial position, and then command the head feeding motor to move the print head to the zero position. When the first signal is then changed to the second signal, the initializing means confirms that the print head has been returned to the zero position, and terminates the initializing operation.
If the second signal is changed to the first signal, the initializing means determines that the ribbon holder is placed in the initial position, and commands the head feeding motor to move the print head to the zero position. When the first signal is then changed to the second signal, the initializing means confirms that the print head has been moved to the zero position, and terminates the initializing operation.
In the printer constructed as described above, the sensor should be designed so that the output signal of the sensor changes depending upon the currently selected position of the ribbon holder even when the print head is located at a position other than the zero position. That is, the sensor should be capable of detecting the position of the ribbon holder even when the print head is not located at the zero position. This requirement results in a complicated arrangement of the sensor.
The above requirement also leads to a problem explained below.
Generally, a sensor includes a first sensed portion which is moved with the print head, a second sensed portion which is moved with the ribbon holder, and a sensing portion for electrically detecting a movement or displacement of the first and second sensed portions and generating an output signal depending upon the detected movement of the sensed portions. The sensing portion is connected to a controller of the printer by a signal line to feed the output signal to the controller. For the sensing portion to be able to detect the currently selected position of the ribbon holder even when the print head is not located at the zero position, the sensing portion is usually attached to a movable member which is moved with the print head. On the other hand, a print head assembly including the print head and such movable member carrying the sensing portion of the sensor is preferably small-sized and light-weight, for increased feeding speed of the print head (increased printing speed) and improved printing efficiency.
In the known printer described above, the sensing portion of the sensor incorporated in the print head assembly inevitably increases the size and weight of the print head assembly, contrary to the above requirement for minimum size and weight for improved printing efficiency. Further, the attachment of the sensing portion of the sensor to the print head assembly leads to reduced efficiency in assembling the print head assembly. This is particularly so, because of a relatively long signal line extending between the sensing portion and a controller. Further, the long signal line pushes up the cost of the sensor, and may lead to reduced resistance of the controller to noises.